This is an open-label (all people involved know the identity of the intervention), single arm, multicenter (when more than one hospital or medical school team work on a medical research study), prospective study (study following participants forward in time) to evaluate the efficacy and safety of OROS methylphenidate in participants with ADHD (behavior disorder originating in childhood in which the essential features are signs of developmentally inappropriate inattention, impulsivity, and hyperactivity). The study duration will be of 12 weeks per participant, which is divided into 2 parts Screening (within 14 days before study commences on Day -1) and treatment (8 weeks and will include titration period [from the initiation of the study treatment to determination of the individual's maintenance dose] and maintenance period [at least 4 weeks after determination of maintenance dose]). Participants will receive initial dose depending on their body weight. OROS methylphenidate hydrochloride (HCL) will be given orally once daily at an initial dose of 18 milligram (mg) for participants below 30 kilogram (kg) and 27 mg for those over 30 kg of body weight. The dose will be increased by 9 mg or 18 mg every week for up to Week 8, followed by a maximum maintenance dose of 54 mg orally once daily up to Week 12 during which the dose can be decreased by 9 mg depending on tolerability. Efficacy will be evaluated by Korean Version, ADHD Rating Scale (K-ARS) total score and Clinical Global Impression)-Severity / Impression rating scale CGI-S/I. Participants safety will be monitored throughout the trial.